Odometer Data Lost on SCP Bus
The U1123 code indicates that the instrument cluster control module (HEC/IC) has detected invalid or missing odometer data messages on the SCP (Standard Corporate Protocol) communication bus. This fault results in a loss of mileage synchronization between modules, which can disable the odometer display and affect dependent systems such as the PATS (Passive Anti-Theft System) and maintenance interval calculations.
1. Commonly Affected Makes
2. Most Common Causes
- Internal failure of the instrument cluster module (HEC/IC)
- Open circuit, short to ground/battery, or high resistance in the SCP bus wiring (+/-)
- Fault in another module on the SCP bus (PCM, BCM/GEM, ABS) that disrupts communication
- Corruption of the cluster's EEPROM memory (stored mileage)
- Low battery voltage or voltage spikes during startup
3. Symptoms You Will Notice
- Odometer display blank or showing dashes
- Mileage reading inconsistent or stuck
- PATS system may not function properly
- Maintenance reminders may be inaccurate
4. Step-by-Step Diagnosis
Check battery voltage and charging system; ensure stable voltage (12.6V+ engine off, 13.5-14.5V running).
Scan all modules on the SCP bus for related codes (e.g., U1xxx codes in PCM, BCM, ABS).
Inspect SCP bus wiring (typically two twisted wires) for opens, shorts, or corrosion; measure resistance and voltage with a multimeter.
If wiring is intact, attempt to reprogram or replace the instrument cluster module.
If other modules have faults, repair those first; a faulty module can block bus communication.
5. Frequently Asked Questions
Is it safe to drive?▼
Yes, the vehicle is generally drivable, but the odometer and related systems (like PATS) may not function correctly. It is recommended to diagnose and repair the issue promptly to ensure accurate mileage tracking and security system operation.
How much does it cost to fix?▼
The cost varies widely depending on the root cause. A simple wiring repair may cost $100-$200, while replacing an instrument cluster can range from $400 to $800 (including programming). If the issue is in another module, costs can be higher.